Silicon-controlled-rectifier (SCR) type dimmers are commonly used for conventional incandescent tungsten filament-type light bulbs. An exemplary SCR dimmer circuit topology is shown in FIG. 1. The circuit of FIG. 1 operates from an alternating current (AC) line input to drive a lighting load (“LOAD”), and includes a diac and a triac (diac and triac are different types of SCR-type devices, also known as thyristors in general), a variable timing resistor R1, a fixed value resistor R2 for minimum value setting, and a timing capacitor C1.
FIGS. 2(a)-2(c) illustrate ideal input and output waveforms of the SCR dimmer circuit of FIG. 1 for an intermediate dimming level set by timing resistor R1. FIG. 2(a) represents the AC line input waveform, FIG. 2(b) represents a leading edge phase cut output waveform, and FIG. 2(c) represents a trailing edge phase cut output waveform. The SCR AC output is directly used to drive the lighting load, which may be a tungsten filament light bulb. Light is produced when the filament is heated by electrical power to glow white-hot. In FIGS. 2(b) and 2(c), θ indicates the on-time of the SCR. A longer on-time generally corresponds to higher electrical power used to heat the filament and brighter illumination. Adjusting the variable timing resistor R1 changes the duration of the on-time as well as the brightness level. The delay duration (“Delay”) in FIGS. 2(b) and 2(c) indicates the SCR off-time during which no electrical power is provided to heat the filament. However, because the on and off cycle time is fast—at double the line frequency—and because fluctuation of the filament temperature is relatively slow due to its slow thermal response, the filament will stabilize at an average temperature that is dependent on the average power determined by the duration of the on-time set by the timing resistor. As a result, the illumination brightness level of the tungsten filament light bulb is steady with no discernable flickering, even though the SCR is switching the power on and off. The SCR dimmer has become the standard apparatus for illumination brightness control in conventional AC incandescent lighting applications.
Due to their favorable energy-saving and environmental considerations, high brightness LEDs are emerging as desirable future lighting devices compared to conventional tungsten filament and fluorescent lights, at least in part due to their energy efficiency. Accordingly, there has been a growing interest in replacing conventional lights with LEDs and in operating LED lighting appliances directly from conventional AC power sources and conventional SCR dimmers.
A tungsten filament produces light emission through its resistive AC conducting nature. An LED, on the other hand, requires forward bias for direct current (DC) conduction to produce light emission, due to its diode nature. Also, unlike the slow thermal response of a resistive tungsten filament, the response time of an LED is less than a microsecond, i.e., nearly instantaneous when compared to the standard AC line frequencies of 50˜60 hertz. As a result, the illumination brightness of an LED will basically follow any intentional or un-intentional fluctuations in the LED drive current.
In order to operate LED lighting appliances from conventional AC power lines and SCR type dimmers, a DC drive circuit may be provided to drive the LEDs using the AC power output from an SCR dimmer. Because slow brightness variations at AC line rates of 50˜60 hertz may be perceived by the human visual system, variations in the drive current at rates below around double the line rates, i.e., below 100˜120 hertz, should be eliminated in order to ensure flicker-free illumination brightness. For brightness variation rates that fall well above AC line rate, the human visual system will average out brightness variations and perceive a steady brightness as long as a steady averaged DC drive current is provided to the LED.
FIG. 3 shows an exemplary switching mode power converter apparatus 300, configured as an AC-to-DC (AC/DC) lighting controller driver for driving an LED from a conventional SCR dimmer output. Switching mode power converter apparatus 300 includes a rectifier circuit 301 for receiving and rectifying the SCR AC dimmer output, and a dimmer loading circuit 302 for providing an appropriate load to ensure proper operation of the SCR. Inclusion of dimmer loading circuit 302 may be desirable because the SCR may require a minimum current for starting and maintaining conduction. Switching mode power converter apparatus 300 also includes a power factor correction (PFC) and electromagnetic interference (EMI) filter circuit 303 to compensate for power factor degradation and to reduce EMI resulting from switching mode power converter operation. Switching mode power converter apparatus 300 further includes a power conversion controller 304 for controlling switching mode power conversion operation, a switching mode power conversion inductor 306, a power switch 307, a current sensing resistor 308, and a power storage capacitor 309. The value of current sensing resistor 308 may be determined according to a desired current sense voltage (VCS) and peak inductor current. Switching mode power converter apparatus 300 is configured to provide power to an LED circuit 305 for illumination. LED circuit 305, switching mode power conversion inductor 306, power switch 307, current sensing resistor 308, and power storage capacitor 309 together constitute an LED device module 310.
Switching mode power converter apparatus 300 functions by switching power switch 307 at frequencies much higher than the line frequency, typically in the range of kilohertz to hundreds of kilohertz, or even megahertz. By appropriately selecting values for inductor 306, capacitor 309, and sensing resistor 308, power conversion controller 304 can provide a switch driving signal (SWITCH_DRIVE) to operate power switch 307 at an appropriately high switching frequency while producing the desired current sense voltage (VCS) to produce a steady average drive current in LED circuit 305. A steady illumination brightness level may be achieved with such a steady average drive current in LED circuit 305, provided that a low variation rate (below around 100 hertz of drive current), which may be visible to the naked eye, is not present.
Although there may be variations in the configuration of the switching mode power converter type lighting controller driver illustrated in FIG. 3, the average drive current in the LED circuit, and consequently the LED illumination brightness level, is generally controlled by tuning the switch driving signal (SWITCH_DRIVE) applied to a gate of power switch 307. Thus, driving and controlling the LED lighting illumination brightness using a light dimmer is performed by generating a control signal dependent on the dimmer output and using that control signal to control switch driving signal SWITCH_DRIVE.
One conventional approach to controlling illumination brightness of an LED circuit is to filter the rectified input voltage from the dimmer to obtain a voltage signal that corresponds to the dimmer input power, and use the voltage signal as a control signal to control the drive signal SWITCH_DRIVE. However, a first problem with this approach is that it requires a relatively large value filter capacitor that may be difficult to integrate onto an integrated circuit. A second problem with this approach is that because the dimmer input is an AC voltage signal, some fluctuations will remain in the voltage signal even when the dimmer is set at a constant setting. Using a line rate fluctuating voltage signal as the control signal may lead to fluctuations in the illumination brightness at line rates to which the human visual system is sensitive. A third problem with this approach is that accurate sensing of the voltage signal over a large voltage range and the accurate control of the drive signal SWITCH_DRIVE using analog circuits in a noisy switching mode power converter system may be difficult.
Another conventional approach is to sense the durations (also referred to as phase angle or duty cycle) of AC waveforms of the input voltage from the dimmer and use the waveform width duration values to provide the control signal. FIGS. 4(a)-4(e) show the voltage waveforms of the outputs of several commercial dimmers. As FIGS. 4(a)-4(e) show, there are large differences among output waveforms of different dimmers in addition to instabilities and irregularities within each output waveform. Because there is no reliable way to determine the amplitudes of these irregularly shaped “pulses,” measuring the pulse widths of these different waveforms is difficult, yielding unreliable and inconsistent results. Cases with multiple “pulses” or “ringings” within one AC cycle further complicate the pulse width measurement. Therefore, in the general lighting application market where many different kinds of commercial dimmers are used, such inconsistent pulse width data prevents production of a consistent control signal in LED lighting control to provide reliable and steady illumination brightness control.